When I Import Two Models Into Blender I Can Only Edit One at a Time
Editing multiple objects

First of all, you can only edit one object at a time.  Normally if you want to edit multiple objects at a time you will have to join them with Ctrl + J.  However there is an addon called multi-edit which allows you to edit multiple objects simultaneously.

The active object

When you go into edit mode with more than one object Blender will keep all other objects selected, but will only allow you to edit the geometry of the active object.  The active object is marked by a slightly lighter orange border than other selected objects.  It is typically the object which you single selected last.  If you do a group select (such as box select with B or brush select with C) the active object does not change.

Check out this post for more on the active object.